This invention relates to gas turbine engines and particularly to means for clearance control of the gap between the seal and rotating blade by controlling the temperature of the engine case by regulating the leakage into and out of the space defined by a double wall construction, which leakage is controlled by the back pressure regulated by the admittance of cool air into said space.
As has become well known in recent years, perhaps the most single advancement that has contributed the most for the fuel economy is the control of the gap between the turbine blades and the adjacent tip seals. A great deal of effort has been expended toward this end which is exemplified by U.S. Pat. Nos. 4,019,320 granted to I. H. Redinger, Jr., D. Sadowsky and P. P. Stripinis on Apr. 26, 1977 and U.S. Pat No. 3,975,901 granted to C. C. Halinger and R. Kervesten on Aug. 24, 1976. In the first patent, air impinges externally of the case to cause it to shrink and position the seal relative to the tip of the rotating blades. The latter mentioned patent directs either hot or cool air with valve means mounted internally of the case to the seal surface to cause the seal to be positioned relative to the tips of the rotating blades.
In the first instance, the use of impingement cooling requires externally mounted spray bars which in certain installations have proven to be efficacious. However, the use of spray bars are additional weight and must be suitably located. In certain installations, the maze of plumbing external of the engine case prohibits the use of spray bars.
The use of internally mounted heat sensitive valve means as suggested in the second mentioned patent has the problem of not only being difficult to fabricate and operate satisfactory during initial installation, but after engine deterioration over the course of its use the repeatability of the valve is adversely affected such that it doesn't respond at the same conditions that it did initially.
I have found that I can obviate the problems noted above by taking advantage of the leakage characteristics of the hardware surrounding the rotating blades, providing a double wall configuration and controlling the back pressure in the space between the double walls so as to regulate the leakage of cool air admitted externally thereto and the hot air in the adjacent gas path. By controlling this leakage this space is subjected to a large temperature differential, namely between the temperature of the cool air which can either be fan discharge or compressor bleed air and the temperature of the engine working fluid medium. Essentially, this allows a great deal of flexibility in the design of the clearance control as it allows the designer to select the seal clearance at any point in the flight envelope. Additionally the response is high allowing a broad range of seal clearance adjustments.